Method for the manufacture of a strip of formable steel

ABSTRACT

A method for the manufacture of a strip of formable steel comprises the steps of (i) forming liquid steel by continuous casting into a slab having a thickness of not more than 100 mm, (ii) rolling the slab in the austenitic region into an intermediate slab having a thickness in the range 5 to 20 mm, (iii) cooling the intermediate slab to below the Ar 3  temperature, (iv) holding the intermediate slab in an enclosure for temperature homogenisation, (v) rolling the intermediate slab into strip, with at least one rolling pass applying a thickness reduction of more than 50%, at a temperature below T t  and above 200° C., wherein T t  is the temperature at which 75% of the steel is converted into ferrite, and (vi) coiling said strip at a temperature above 500° C. Advantages of simplicity of the method and the plant required for it are obtained.

FIELD OF THE INVENTION

This invention relates to a method for the manufacture of a strip offormable steel.

DESCRIPTION OF THE PRIOR ART

EP-A-370575 describes a method for making formable steel strip in whichliquid steel is formed in a continuous casting machine into a thin slabwith a thickness smaller than 100 mm, and, with use of the casting heat,the steel slab is rolled in the austenitic range into an intermediateslab. The intermediate slab is cooled to a temperature below Ar₃ and, ata temperature below T_(t), at which 75% of the material is convertedinto a ferrite, and above 200° C. is rolled into the strip. A drawbackof this method is that, to use it for manufacturing a steel strip withgood forming properties, it requires a complicated plant, not leastbecause of the proposed large reduction in the ferritic range and therecrystallisation furnaces needed for obtaining a desired structure.Related methods, less relevant to the present discussion are disclosedin EP-A-306076 and EP-A-504999.

SUMMARY OF THE INVENTION

One object of the invention is to provide a method which can be carriedout continuously and with a simple plant, and by which a steel stripwith good forming properties can be obtained.

In one aspect the invention provides a method for the manufacture of astrip of formable steel, comprising the steps of, in the followingorder:

(i) by continuous casting, forming liquid steel into a slab having athickness of not more than 100 mm,

(ii) rolling the slab, while it is still hot from its casting and in theaustenitic region, into an intermediate slab having a thickness in therange 5 to 20 mm,

(iii) cooling the intermediate slab to a temperature which is below theAr₃ temperature of the steel,

(iv) holding the intermediate slab in an enclosure for temperaturehomogenisation thereof,

(v) rolling the intermediate slab into strip, with at least one rollingpass applying a thickness reduction of more than 50%, the intermediateslab being below a temperature T_(t) at which 75% of the steel isconverted into ferrite and above 200° C.,

(vi) coiling the strip at a temperature above 500° C.

This method requires a smaller number of process stages. By this methodgood forming properties may be achieved without the steel striprequiring recrystallisation annealing. The finishing train by which theintermediate slab is rolled into the strip may be of simple constructionbecause only a relatively small reduction is made. Another advantage isthat, because the mean temperature during the entire process is onaverage higher, the rolling forces are on average lower. The plant forcarrying out the method may then be built lighter and with a lowerinstalled capacity.

Another advantage is that storage from the homogenisation can allowsufficient time for precipitation to TiC in the case of IF steel.

Preferably the steel strip is coiled at a temperature above 600° C.So-called self-annealing then occurs in the coiled coil as a consequenceof the heat content of the steel strip.

Another advantage of the relatively thin intermediate slab is that thethickness reduction in the ferritic region is relatively small and thatthe relationship between exit speed and entry speed is thus relativelylow. The exit speed may be set at around a conventional value of 600m/min, for which technology is available. Because the intermediate slabis relatively thin the entry speed is still high. The advantage of thisis that the time that the intermediate slab is exposed to thesurrounding atmosphere, thus allowing oxide to form on its surface, isbrief. Therefore, with the method it is possible to make a strip withlittle oxide space. The entry speed is preferably >0.8 m/s.

Improved deformation properties of the steel strip are obtained becausethe intermediate slab undergoes at least one pass having at least 50%reduction in the ferritic region. Such deformation is quite adequate forintroducing recrystallisation. In addition the advantage is achievedthat, with such deformation, the temperature drop of the steel as aconsequence of heat loss to the surroundings and to the mill rolls maybe considerably compensated for by the deformation energy introducedinto the steel during rolling. By applying this reduction, virtually noheat loss occurs in the rolling train, so that the intermediate slab canbe rolled in the first mill stands at relatively low temperatures andless oxide will form.

The reduction in this pass is preferably less than 60%, more preferablyless than 55%. In the case of large-reduction passes non-linearitiesstart to play a part and lead to the problem that the rolled steel isdifficult to control in and after the rolling apparatus.

Especially effective is a preferred embodiment of the method in whichlubrication rolling is carried out in at least one pass in the ferriticregion. Lubrication rolling reduces the rolling forces, achieves a goodsurface condition and the deformation applied by the pass is uniformalydistributed across the cross-section, so that homogeneous materialproperties are obtained. This lubrication rolling pass is optionally thepass in which more than 50% reduction is performed.

A crystal structure and a crystal size distribution which are favourablefor ferritic rolling are achieved if the cast slab in the continuouscasting is reduced in thickness with its core still liquid.

The steel strip is preferably rolled to a thickness less than 1.0 mm.

The method according to the invention can be carried out with a plantfor the manufacture of steel strip, comprising

(a) a continuous casting machine for casting a steel slab,

(b) a furnace apparatus arranged for receiving the steel slab cast inthe continuous casting machine (optionally with thickness reduction ofthe solidified slab prior to entry to the furnace apparatus), foradjusting the temperature of the steel slab, the furnace apparatushaving an entry port and an exit port for the slab and an enclosed pathfor the slab from the entry port to the exit port,

(c) a coiling apparatus for receiving the steel slab from the furnaceapparatus, coiling the slab and subsequently uncoiling the slab, thecoiling apparatus having an enclosure providing an enclosed space inwhich the slab is coiled and an entry port for entry of the slab intothe enclosed space,

(d) rolling apparatus for receiving the steel slab uncoiled from thecoiling apparatus and rolling the slab into strip of a desiredthickness, and

(e) means for providing a non-oxidising gas atmosphere in the furnaceapparatus at the path thereof and in the enclosed space of the coilingapparatus,

wherein the exit port of the furnace apparatus is gas-tightly connectedto the entry port of the coiling apparatus.

Such an apparatus and its advantages and specific embodiments aredescribed in the International patent application "Plant for themanufacture of steel strip" with the same filing date as the presentapplication and in the name of the same applicant, with reference no. HO848. The content of that application is deemed to be included in thepresent application by this reference.

By this plant there is achieved the effect that from the time when theslab runs into the furnace apparatus until the time it is conveyed outof the coiling apparatus, the slab does not come into contact with theoutside air, but rather it is continually surrounded by a gaseousatmosphere of a non-oxidizing composition. For this purpose the gaseousatmospheres in the furnace apparatus and in the coiling apparatus may bethe same or different.

The gas atmosphere provided in the furnace apparatus and the coilingapparatus is substantially non-oxidizing, though inevitably it mayinclude a small amount of oxygen due to leakage of air. Preferably it isbased on nitrogen, although an inert gas such as argon may be used ifits high cost allows. The nitrogen may contain additive for inhibitingnitriding of the steel surface, as is known in the process of batchannealing of steel. The gas atmosphere may contain water vapour.

Typically the furnace apparatus is built as an electric furnace inwhich, by means of resistance or inductive heating, energy is suppliedto the slab, so that in any event the surface of the slab is heatedagain after having cooled as a consequence of the descaling by highpressure water sprays and because of heat loss to the surroundings. Inthe case of conventional plants, during this heating the surface isexposed to the normal outside atmosphere along a relatively greatdistance and thus for a relatively long time, so that an oxide scaleagain forms on the surface, which under these conditions is a thin,tenacious layer which in practice cannot be completely removed withavailable very high water pressures and which ultimately must be removedby pickling.

The furnace apparatus may be employed only for homogenizing thetemperature of the steel slab, or may be arranged to alter at least thecore of the slab in temperature.

In the plant the slab is prevented from coming into contact with theoutside atmosphere as it passes through even a relatively long furnaceapparatus, so that oxide scale thereby forming on the outer surface ofthe slab is minimized.

As stated, the coiling apparatus is provided an enclosure, i.e.screening means, for maintaining the desired gaseous atmosphere in thecoiling apparatus. In the case of a conventional plant, the slab iscoiled at a relatively high temperature in the coiling apparatus andstored there for some time for temperature homogenising or for waitingfor further processing in the rolling apparatus. The slab is preventedfrom still oxidising or oxidising further during its stay in the coilingapparatus.

As mentioned the exit of the furnace apparatus is coupled essentiallygas-tight to the coiling apparatus. Preferably the furnace apparatus andthe coiling apparatus are detachably coupled to one another.

Other possibilities are provided with an embodiment of the plant inwhich the furnace apparatus is provided with cooling means for coolingthe gas of the gaseous atmosphere. With this embodiment it is possibleto cool the slab, if desired following roughing in the austeniticregion, in a conditioned gaseous atmosphere down to the ferritic regionpreferably above 200° C. or to the lower part of the two-phaseaustenitic-ferritic region, and to coil the slab at such a temperaturewithout a harmful amount of oxide forming on the surface. When still inthe temperature region indicated, the slab may be rolled in the rollingapparatus into the steel strip of a desired thickness. This embodimentthereby opens up the possibility of making a formable steel strip havingcold strip properties as regards forming behaviour and surface quality,in a very compact installation. Where still higher demands are placed onthose properties, the strip may, if desired, be further processed in theconventional manner, whether or non in-line, or in a followingcontinuous process.

Another feature which provides greater flexibility in use is that thecoiling apparatus is provided with a mandrel onto which the coil can becoiled. The crop end of a slab, whether or not subjected to roughing, isclamped onto the mandrel and then coiled in the coiling apparatus intothe coil in a path determined by the mandrel. This forced path makes itpossible to coil a wide range of thicknesses reliably. This achieves agreat freedom in the part of the process taking place prior to coiling,and it is also possible to coil thin, rolled slabs. Such slabs have arelatively large exposed surface. With the plant this surface isscreened from oxygen from the outside atmosphere. Consequently it ispossible to profit from the plant to the maximum.

INTRODUCTION OF THE DRAWINGS

The method according to invention will be illustrated in the followingby means of a description of a non-limitative example of a plant forcarrying out the method with reference to the drawings.

In the drawings:

FIG. 1 is a schematic top-view of a plant for carrying out the method ofthe invention, and

FIG. 2 is a schematic side-view of the plant of FIG. 1.

DESCRIPTION OF THE EMBODIMENT

FIG. 1 shows a continuous casting machine 1 for two strands. Thecontinuous casting machine 1 comprises a ladle turret 2 in which twoladles 3 and 4 can be accommodated. Each of the two ladles can containapproximately 300 tons of liquid steel. The continuous casting machineis provided with a tundish 5 which is filled from the ladles 3 and 4 andkept filled. The liquid steel runs out of the tundish into two moulds(not drawn) from where the steel, now in the form of a partiallysolidified slab with its core still liquid, passes between the rolls ofcurved roller tables 6 and 7. For some grades of steel t can be anadvantage to reduce the steel slab in thickness in roller tables 6 and 7while its core is still liquid. This is known as squeezing.

Descaling sprays 8 are located on the exit side of the two roller tables6 and 7, by which oxide scale is sprayed from the slab with a waterpressure of approximately 200 bar. Starting with a cast thickness of forexample approximately 60 mm, the slab typically still has a thicknessfollowing squeezing of approximately 45 mm. By the 3-stand roll trains 9and 10 the slab is further reduced to a thickness ranging from 10 to 15mm. If desired the head and the tail may be cut off the slab by theshears 11 and 12, or the slab sheared into parts of a desired length.

Instead of casting a thin slab with a thickness of less than 100 mm, itis also possible to cast a thicker slab and by means of rolling, inparticular by means of reversible rolling, to reduce the thickness ofthe slab to a value ranging from 10 to 15 mm.

This apparatus is used to make a ferritically rolled strip. In thisapplication the slabs are preferably rolled in rolling trains 9 and 10to a thickness of approximately 10 mm. Furnace apparatuses 13 and 14 areused primarily as cooling apparatus, possibly in combination with extraheating to compensate for heat losses, or to heat the slab locally asrequired. In addition to, or instead of, the furnace apparatus, coolingusing water or air may be employed. To obtain the cooling effect the gasis sucked from the furnace apparatus through suction line 15, arrangedinto a desired composition and cooled in the conditioning apparatus, andthen conveyed back into the furnace apparatus through line 21. Bothfurnace apparatuses are equipped with such a conditioning apparatus. Asuitable value for the temperature of the slab on exiting the furnaceapparatus is 780° C.

The slab is coiled in the manner described above into a coil which ismoved to position E stored in one of the coiling apparatuses. Thisallows temperature homogenization in the coiled slab.

The furnace apparatuses 13, 14 are in the form of encloses and areprovided with conditioning means for creating and preserving a desirednon-oxidizing gaseous atmosphere in the furnace apparatus. In theembodiment shown the conditioning means of a furnace apparatus comprisea suction line 15, a pump 17, gas metering and gas scrubbing means 19and a supply line 21 along which the gas is pumped into the furnaceapparatus. If desired the gas metering and gas scrubbing means 19 mayalso comprise a gas heating apparatus for compensating for any heatloss. Thus, heat exchangers can be employed to control the gastemperature, using gas combustion to supply heat, and water for cooling.

The furnace apparatus is provided on its entry and exit sides with ports23, 25 having sealing means to substantially prevent any undesiredpenetration of gas from the surrounding atmosphere. A suitable value forthe temperature of the reduced slab on exiting the furnace apparatus is780° C. The furnace apparatus is coupled essentially gas-tightly to thecoiling apparatus 27, which coiling apparatus 27 itself comprises anessentially gas-tight enclosure in which the slab is coiled into a coil.The coiling apparatus is preferably provided with a mandrel 29 whichsupports the coil as it is being coiled.

In this embodiment, the gas atmosphere provided in the furnace apparatusalso enters the coiling apparatus when the latter is connected to it.Alternatively both the furnace apparatus and the coiling apparatus maybe provided with conditioning means, as described above, for providingthe desired atmosphere.

As appropriate, virtually synchronously with coiling of a slab ontocoiling apparatus 27, a slab cast on the other strand is coiled incoiling apparatus 28 provided with a mandrel 30 (not drawn). Coilingapparatuses 27 and 28 and furnace apparatuses 13 and 14 are eachprovided with sealing means 33, 35, 34, 36 respectively, by which thecoiling apparatuses and the furnace apparatuses may be sealed foruncoupling, so that following uncoupling no gas can penetrate from theoutside atmosphere and the gaseous atmosphere in the coiling apparatusesand the furnace apparatuses remains preserved.

The sealing means for the ports of the furnace apparatuses and thecoiling apparatuses are suitably steel flaps, biased to the closedposition, or they may be doors which are driven. To minimize gasleakage, flexible curtains may additionally be provided.

As soon as the coiling apparatus 27 is filled with a slab coiled into acoil, this coiling apparatus 27 is uncoupled from the furnace apparatus13 and driven from position A (see FIG. 1) past position B to positionC. At position C there is a turnstile 31 (not drawn) by which atposition C the coiling apparatus may be rotated through 180° around avertical axis. Following rotation the coiling apparatus is driven pastwaiting position D to entry position E. As a coiling apparatus travelsfrom position A to position E, an empty coiling apparatus is driven fromposition E to a turnstile 37 at position F. Following rotation through180° around a vertical axis by the turnstile 37, the coiling apparatusis driven past position G to the starting position A and there it isready for taking up a fresh slab.

A corresponding working method is applicable for the second strand,whereby the coiling apparatus 28 filled with a coil is driven fromposition B to position C and following 180° rotation to position D. Thecoiling apparatus stays parked in this position until a coilingapparatus which is currently uncoiling, for example coiling apparatus27, is empty at position E and driven off to the now vacated position F.As soon as coiling apparatus 28 leaves position B, an empty coilingapparatus from position I, following rotation through 180° around avertical axis by means of a turnstile 38, is moved via position K totake up the position of the coiling apparatus 28 now driven off. The newslab fed out of the furnace apparatus 14 can be coiled in the emptycoiling apparatus. Devices, preferably electrical current conductors(not shown), are fitted along the paths over which the coilingapparatuses travel for providing power for internally heating thecoiling apparatuses according to need. For this purpose, the coilingapparatus contains electrical heaters for heating the coils and contactsfor pick-up of power from the fixed conductors. Path B, C, D, E iscommon and used as described by coiling apparatuses of both strands.Position C has a rotation facility and position D is a waiting positionin which a coiling apparatus filled with a coil is ready to be moved toposition E as soon as it becomes free. Positions C and D may be swappedor may coincide.

In the manner described, a coiling apparatus 27 arrives at position Ewith its sealing means 33 closed and filled with a coil with atemperature of approximately 780° C. After the sealing means 33 havebeen opened the extremity of the outer winding corresponding to the tailof the coiled slab is fed into the rolling train. If desired the head bybe cut off by crop shears if it does not have a suitable shape orcomposition for further processing. Should some oxide still haveoccurred, this can then be removed easily using the high pressure spray42. In practice oxide formation will be negligible because the slab hasbeen almost constantly in a conditioned gaseous atmosphere. Because thecoiling apparatus rotates through 180°, its original infeed which is nowthe outfeed can be brought up very close to the entry of the rollingtrain. This also minimizes oxide formation.

In the example shown, the rolling train 40 is provided with four millstands and is so designed that the slab can be rolled in the ferriticrange. For controlling thickness, width and temperature, a measuring andcontrol apparatus 43 may be incorporated in the rolling train, after orbetween the mill stands.

As described above, the apparatus achieves the effect that less oxideforms as the slab and the strip are being processed. Because of this andbecause of the lower entry speed in the last rolling train 40 which thisachieves as an additional advantage, it is possible to attain a smallerthan conventional finished thickness of the hot rolled steel. Exitthicknesses of 1.0 mm and less from the rolling train 40 can be attainedwith the plant described.

Following any desired cutting off the crop end with shears 41, and ifdesired, following oxide removal by means of high pressure sprays, theferritic slab is rolled in the ferritic region in the rolling train 40to a finished thickness which, as is conventional, ranges between 0.7 mmand 1.5 mm. For most steel grades further cooling is not necessary andthe ferritic strip can be coiled into a coil on the coiling apparatus 46which may be placed at a short distance after the rolling train.

In particular, one of the mill stands of the rolling train 40, beingpreferably not the first mill stand, applies a thickness reduction ofthe slab of more than 50%, preferably not more than 55%. One of the millstands of the train 40 applies lubrication rolling; again this ispreferably not the first mill stand.

Coiling of the finished strip in the coiling apparatus 46 is at over500° C., preferably over 600° C.

Therefore, using the plant in this manner it is possible using thecasting heat to manufacture in a successive series of process stages aferritically rolled steel strip with good properties in particular interms of the surface quality. External heating after casting may beavoided (except any heat generated by the rolling).

The proposed paths of movement of the coiling apparatus between thefurnace apparatus and the rolling train allow for a very compactconstruction, in particular in a direction transverse to the directionof passage of the steel through the apparatus. This makes it possible tocast simultaneously two strands from just one tundish while using justone ladle turret. This achieves a considerably reduction of thefinancial capital which needs to be invested in the plant.

We claim:
 1. A method for the manufacture of a strip of formable steel,comprising the steps of, in the following order:(i) by continuouscasting, forming liquid steel into a slab having a thickness of not morethan 100 mm, (ii) rolling said slab, while still hot from its castingand in the austenitic region, into an intermediate slab having athickness in the range 5 to 20 mm, (iii) cooling said intermediate slabof step (ii) to a temperature which is below the Ar₃ temperature of thesteel, (iv) holding said cooled intermediate slab in an enclosure fortemperature homogenization thereof, (v) rolling said held intermediateslab into a strip, with at least one rolling pass applying a thicknessreduction of more than 50%, said held intermediate slab during rollingbeing at a temperature in the range from above 200° C. to below atemperature T_(t) at which 75% of the steel is converted into ferrite,(vi) coiling said strip at a temperature above 500° C.
 2. A methodaccording to claim 1 including in step (i) a thickness reduction of thecast steel while its core is still liquid.
 3. A method according toclaim 1 wherein in said step (iv) said enclosure is provided by at leastone of a furnace apparatus containing said intermediate slab and acoiling apparatus in which said intermediate slab is coiled.
 4. A methodaccording to claim 1, wherein from the continuous casting of step (i) tothe coiling of step (vi) the steel as a whole is not subjected tore-heating, apart from any heat generated by the rolling.
 5. A methodaccording to claim 1 wherein the thickness of the strip produced in step(v) is in the range 0.7 to 1.5 mm.
 6. A method according to claim 1wherein step (v) includes at least one pass of lubrication rolling.
 7. Amethod according to claim 1 wherein in step (iv) said intermediate slabis at a temperature below T_(t) and above 200° C.